Alternative way to achieve cancelable promise

[Kagami Rosylight]

>So by default, promises must not be cancellable.

With some API change now the proposal allows this, similar to other token based proposals:

```
// if you don’t want to allow other consumers cancel this call
// for security reasons or whatever, you can disallow it by
// creating chain explicitly:
function foo() {
  const chain = new CancelableChain();

  // here myFetch() will register to cancellation chain and will be canceled
  // by chain.cancel()
  return myFetch(“url”, { chain });
}
```
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